1. Field of the Invention
The invention relates to aspheric lens structures, and more particularly to lens structures with dual aspheric surfaces and fabrication methods thereof.
2. Description of the Related Art
Digital cameras utilizing high-resolution electronic imaging sensors typically require high resolution optical components such as aspheric lenses. Aspheric lenses have some optical advantages, but cannot be easily produced by traditional glass grinding and polishing techniques. Aspheric lenses with all glass elements may be large and excessively expensive for use in compact digital cameras or accessories built into a cellular phone.
Aspheric elements are typically produced by molding plastics or low melt temperature glasses. While molded plastic elements are inexpensive to produce, the level of precision of the lenses is not always sufficient for high-resolution cameras, particularly if a plastic element is used primarily as a focusing element. Further, a conventional aspheric lens with a single aspheric surface is produced by molding a resin on a plate and is then replicated to create an array of aspheric lens on the plate for wafer level package application. The ability to mass produces molding lenses is limited, thus, manufacture thereof is expensive.
FIG. 1 is a schematic view of conventional aspheric lens fabricated by molding. In FIG. 1, a transparent resin 3 is injected on a plate 1. A die 2 moves toward the substrate to mold the transparent resin 3 into a lens with an aspheric surface 3a on the plate 1, the die then returns to its original position. An array of aspheric lens 3 on the plate 1 is replicated by repeating the described molding procedure for wafer level package application.
FIG. 2 is a schematic view of another conventional hybrid aspheric lens fabricated by transferring an aspheric composite layer on a convex lens. Referring to FIG. 2, conventional hybrid lenses, in which an aspheric composite layer is formed on a single lens of optical glass or lens plate serving as a parent material, are commercially practical as a means for forming an aspheric lens at relatively low cost. The forming die 10 has a smoothed concave transfer face 20 for forming an aspheric composite layer 30 thereon. After a predetermined amount of ultraviolet-curing resin 30, which is the material of the aspheric composite layer, is poured on the transfer face 20, a convex lens 40 made of optical glass is placed on and fixed to the forming die 10 via a support frame 50. The ultraviolet-curing resin 30 thereby fills the entire space between the transfer face 20 and the convex lens 40. The ability to mass produce aspheric composite layer transferred to a convex lens is limited and thus expensive to manufacture.